This invention relates generally to fuel ignition devices used in internal combustion engines and more specifically to glow plugs used in diesel engines to ignite the air fuel mixture and a method for making such glow plugs.
The present invention relates to glow plugs of the type used in diesel engines for vehicles for igniting the air fuel mixture quickly. One of the major objections to the use of diesel engines in automotive applications has been the delay in starting the engines, which has resulted from the time required to heat the glow plugs to the starting temperature. As a consequence, there has been a continuing attempt by designers and manufacturers to reduce the time required to heat the glow plug to incandescence or the ignition temperature which is on the order of 1000.degree. C.
The typical glow plug includes a protective sheath which is closed at the outer end and sealed at its inner end to a support portion which is threadedly received in an opening in the cylinder head of the engine, to mount the sheath in a location in the engine cylinder to ignite the air-fuel mixture. The sheath may be heated by a coiled resistance heater supported coaxially within the sheath by compacted magnesium oxide, which electrically insulates the coiled heater from the sheath. The outer end of the heater is connected to the tip of the sheath and the inner end is connected to an electrical terminal, which is insulated from the support portion. The terminal and the sheath are connected to a twelve volt DC supply voltage to supply current to heat the coiled heater element.
In an effort to decrease the time to heat the glow plug to the ignition temperature, many alternative variations have been made in the basic elements described above. The high temperatures and pressures of 1100.degree. C. and 400 psi within the engine cylinder have limited the flexibility in design approaches that might be taken. In addition, the combustion gases within the engine cylinders tend to be very corrosive. The metallic sheath has a tendency to decrease the effectiveness of the heater in attaining the ignition temperature, since it presents a significant mass or heat sink to be raised in temperature by the heater. However, the hostile nature of the environment within the engine cylinder has necessitated the continued use of the protective sheath. Attempts made at providing glow plugs with exposed heating elements, unprotected by a sheath have been unsuccessful due to the short life of the heating elements. The sheath is typically formed of a high temperature, corrosive resistant alloy such as a nickel, chromium, iron alloy sold under the trademark Inconel.
Alternative designs for the coiled heating element have been employed to reduce heat-up times. Dual elements having one resistance heater portion and a control portion made of a material having a positive temperature coefficient of resistance have been used in glow plugs distributed commercially and disclosed in issued patents. Examples of patents directed to dual element glow plugs are Bauer U.S. Pat. No. 4,556,781; Murphy et al. U.S. Pat. No. 4,423,309 and Glauner et al. U.S. Pat. No. 4,211,204. The control resistance portions are typically made of nickel or some other material which increases in resistance by a factor of 4 or 5 when heated from room temperature to a temperature of 1000.degree. C. Such control resistance permits the glow plug to be designed to provide a very high initial current which is decreased as the glow plug reaches ignition temperature so as to prevent damage from the sustained high current level. In these dual element plugs, the heater resistance was typically at the outer end of the sheath and the control resistance was toward the inner or mounting end of the sheath.
Other approaches involved concentrating the heating element toward the end of the sheath to minimize the heat loses and increase the effectiveness of the heating element. The patent to Testerini U.S. Pat. No. 3,158,787 is an example of a glow plug having such a heating element concentrated toward the tip of the sheath. The various approaches described above and others have been partially successful in lowering heat-up times for glow plugs, but the times are still generally in excess of ten seconds. One of the problems with existing commercially available glow plugs is that they start to glow or reach incandescence toward the middle of the sheath and not at the outer end. This condition is undesirable from two standpoints.
First, when the concentration of heat is between the ends of the sheath, the plug usually must have the total length of the sheath heated before the ignition temperature is achieved. Since the plug mounting conducts heat away or is a heat sink, the amount of heat required to reach ignition is greater, requiring a longer time. It would be preferable to heat the tip of the glow plug to incandescence first, to minimize heat loss and shorten the heat-up time.
The second problem associated with the failure to heat the tip of the glow plug relates to the present trend toward eliminating precombustion chambers for mixing air and fuel, and the trend toward direct injection in which the fuel is injected into a restricted space between the face of the piston and the adjacent wall of the cylinder head. It has been common in the past to have diesel engines formed with separate chambers into which the fuel was injected, mixed with air, and ignited. The glow plug for igniting the air-fuel mixture would often extend an inch or more into this chamber. Many current engines are designed with reduced size fuel mixing chambers or with the fuel injected directly into the area at the face of the piston.
These new designs leave much less space for the location of the glow plug. It is important to maintain the glow plug separated enough from the fuel injector so that the fuel is not sprayed directly on the sheath, since that tends to lengthen the heat-up time for the glow plug. As a consequence of the limited spaced available for mounting the glow plug, the current size of the plugs, and the fact that heat-up begins at the midpoint of the sheath rather than at the tip, renders the current glow plugs less than satisfactory. It would be preferable to have a shorter sheath in the glow plug and have the heat-up occur first at the tip rather than at the midpoint. With such conditions, less length of the sheath would be required to extend into the combustion area and the effective ignition part of the glow plug could be positioned in the optimum location to ignite the air-fuel mixture.
Various approaches have been followed in connecting the coiled heating element to the tip of the sheath to complete the heating circuit for the glow plug. U.S. Pat. No. 4,477,717 to Walton shows the axially extending end of the heating element extending through an opening in the end of the sheath when it is attached. The Mann U.S. Pat. No. 4,281,451 discloses the use of sintered metal to connect the element and the sheath. The German Patent No. DE 3,003,799 discloses a coiled heating element that is welded through an opening in the sheath to connect the element and sheath and seal the tip of the sheath. The German patent is directed to a dual element glow tube and at the outer end of the heating element where it is welded to the sheath, a number of the heater coils are engaged to short them out and reduce the heating effect in the outermost coils of the heating element.